1. Field of Invention
The invention is directed generally to gateways, bridges, and routers and more particularly, to a method and apparatus for interfacing a local area network (LAN) with a Integrated Services Digital Network (ISDN).
2. Description of Related Art
Networks can be organized in a number of ways, either globally covering great distances or they can be local area networks (LAN) covering relatively short distances. A LAN ordinarily will consist of computers distributed within an office, a building or a campus. Generally speaking, a LAN includes: 1) a high speed transmission medium, typically metallic or fiber optic, for connecting each of the devices to the LAN; 2) the ability to transmit a message on the transmission medium directed to a single device; and 3) a means known as "broadcast" in which all devices connected to the LAN medium can receive a message transmitted on the medium. Several standards for the implementation of LAN devices and systems have been established by the Institute of Electrical and Electronic Engineers LAN/MAN Standards Committee (Project 802).
The physical length of the transmission medium and the total number of devices connected thereto are typically limited on a LAN due to the physics of high speed transmission systems. Bridges and routers are devices used to connect multiple LANs to provide communications between individual LANs and to construct large networks that transcend the technical size limits of a single individual LAN. When the individual LANs to be interconnected are at geographically remote locations, bridges and routers are used in pairs, one at each site, to provide a path for data to flow from one LAN to another.
An all-digital telephone network, known as the Integrated Services Digital Network (ISDN), has become a potential substitute for the private long distance lines currently used by bridges and routers. ISDN provides relatively high speed digital transmission service on an "as needed" basis, and is different from LAN transmission media in that it is a circuit switched transmission media which provides a point-to-point transmission service on an intermittent basis. ISDN enables local area networks to be interconnected with each other to form a wide area network. The ISDN network architecture is based on standards set by International Telecommunications Union (ITU) with standards in the United States largely driven by Bellcore. Two types of ISDN service are available: Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI). BRI delivers two B-channels, each having a capacity of 64 Kbps and a 16 Kbps D-channel. PRI provides twenty three B-channels of 64 Kbps and a 64 Kbps D-channel. The D-channel is used for signaling between the central office switch and terminating equipment which could be a telephone set, personal computer, videoconferencing set or other device. The B-channels are used for any kind of service including voice data and video.
Both IEEE 802 LANs and ISDN are structured by architecture closely following the Open Systems Interconnection (OSI) Seven Layer Reference Model. The OSI model decomposes a communication system into seven major components or layers which are defined by international standards. The OSI model is concerned with the interconnection between systems, i.e., the way they exchange information, and not with the internal functions that are performed by a given system. Communications between systems are organized into information that is exchanged between entities at each layer. The mechanism for communication between two systems at a single layer is referred to as a protocol, i.e., "a layer x protocol."
The first layer is known as the physical layer and is responsible for the transmission of bit streams across a particular physical transmission medium. This layer involves a connection between two machines that allows electrical signals to be exchanged between them.
The second layer is the data link layer (DLL), and is responsible for providing reliable data transmission from one node to another and for shielding higher layers from any concerns about the physical transmission medium. It is concerned with the error-free transmission of frames of data.
The third layer, the network layer (NL), is concerned with routing data from one network node to another and is responsible for establishing, maintaining, and terminating the network connection between two users and for transferring data along that connection. There is normally only one network connection between two given users, although there can be many possible routes from which to choose when the particular connection is established.
The fourth layer is the transport layer (TL), and is responsible for providing data transfer between two users at an agreed on level of quality. When a connection is established between two users, the transport layer is responsible for selecting a particular class of service to be used, for monitoring transmissions to ensure the appropriate service quality is maintained, and for notifying the users if it is not.
The fifth layer is the session layer, and it focuses on providing services used to organize and synchronize the dialog that takes place between users and to manage the data exchange. A primary concern of the session layer is controlling when users can send and receive, based on whether they can send and receive concurrently or alternately.
The sixth layer is the presentation layer, and is responsible for the presentation of information in a way that is meaningful to network users. This may include character code translation, data conversion or data compression and expansion.
The seventh layer is the application layer, and it provides a means for application processes to access the system interconnection facilities in order to exchange information. This includes services used to establish and terminate the connections between users and to monitor and manage the systems being interconnected and the various resources they employ.
Network devices such as bridges, routers and gateways each operate at a separate one of the seven OSI layers. Bridges are data-link layer devices that connect similar networks together, e.g., Ethernet to Ethernet. Routers are network layer devices that perform routing by maintaining a routing table in each host. Gateways operate at the higher OSI Layers, carrying out protocol conversion and other connectivity functions.
Information transferred across networks includes headers and payload, where the headers contain information specific to one of the corresponding seven layers of the OSI model. Headers and payload on a LAN are referred to as a packet. Headers and payload on an ISDN are referred to as frames. Until recently network traffic on either a LAN or ISDN network comprised packets/frames with up to seven headers, and a payload. The headers contained information specific to each of the seven layers of the OSI model. The payload contains the audio, video, or data being transferred. On the LAN the structure of headers and payload is specified by the respective IEEE LAN standard such as 802.3, 802.5 etc.. These standards are heriniafter referred to as 802.x. On the ISDN side the structure of the headers and payload is specified by the High-Level Data Link Control (HDLC) standard promulgated by the International Standards Organization (ISO).
New standards for televideo transmissions promulgated by International Telecommunications Union have significantly altered the relative symmetry between frame and packet construction on respectively ISDN and LAN networks. The International Telecommunications Union has proposed protocols for packet and frame transmission over packet switched and circuit switched networks for televideo conferencing. The International Telecommunications Union standards for televideo conference traffic on a point-to-point network such as ISDN are described in specification H.320 entitled, "Narrow-Band Visual Telephone Systems and Terminal Equipment" revised on March 1996. The corresponding standard for televideo conference traffic on a LAN is described in International Telecommunications Union's specification H.323 entitled, "Visual Telephone Systems and Equipment for Local Area Networks which Provide a Non-Guaranteed Quality of Surface" revised November 1996. The H.320 standard addresses the issues of achieving a natural relationship between the visual image and the oral message for the viewer/listener. An additional benefit to the specification is that in contrast to preexisting DLL frame protocols for communication such as HDLC, an H.320 frame has a higher percentage of data as opposed to header bits in each frame than does HDLC. Thus, processing time is reduced and data throughput is increased for televideo conferencing traffic transmitted in H.320 frames as opposed to identical traffic transmitted in HDLC frames.
Unfortunately existing gateways, routers and bridges require dedicated transmission lines for HDLC and for H.320 traffic, because the protocols differ so significantly at the DLL level. What is needed is a device which can integrate the handling of the H.320 frames and the HDLC frames.